This invention relates to improvements in or relating to the method of two-stage combustion of pulverized coal by means of a pulverized coal burner capable of reducing the amount of NOx produced by combustion.
When pulverized coal is combusted by a pulverized coal burner, NOx of an amount in the range between 500 and 600 ppm is produced as a result of complete combustion of the pulverized coal. Release into the atmosphere of the exhaust gas containing a large amount of NOx should be avoided from the point of view of avoiding air pollution. To this end, it has been earnestly desired that the amount of NOx produced by the combustion of pulverized coal be reduced as much as possible.
In our experience, two-stage combustion and exhaust gas recycling have effect in reducing the amount of NOx produced by the combustion of pulverized coal. In a two-stage combustion method of the prior art, it has hitherto been customary to burn pulverized coal in two steps by limiting the proportion of air for combustion in the second stage to 20% or less at most of the air required for combustion and by mixing the exhaust gas in a proportion of 20% or less or most. The reason for adopting these proportions is that if the proportion of air for combustion in the second stage is increased above the aforesaid limit, non-combusted air would increase in amount and if the exhaust gas is increased above the aforesaid limit, the combustion as a whole would become unstable. In the prior art, the amount of the NOx produced by the combustion of pulverized coal has only been reduced to the range between 200 and 300 ppm. With exhaust gas being added in a proportion of about 20%, the air necessary for combustion has a demand for a relatively high rate of O.sub.2 and consequently diffusion of O.sub.2 to the combustion zone occurs at high speed, so that the pulverized coal is essentially combusted at high temperature and at high speed with high O.sub.2. The result of this is that the pulverized coal burns in non-homogeneous diffusion combustion, so that the flame has a high temperature and the amount of NOx produced increases due to rapid combustion of the pulverized coal with high O.sub.2. Rapid combustion of the pulverized coal also produces non-combusted components, such as hydrocarbons which are activated intermediate products responsible for denitration reaction, NH.sub.3, HCN and CO. These non-combusted components are combusted and disappear as soon as they are produced, so that they do not help in decomposition of NOx and the amount of the latter does not shown a reduction.